1. Field of the Invention
The present invention relates to electrostatic discharge protection devices and, more particularly, to a silicon controlled rectifier with enhanced turn-on time for protecting an internal circuit from electrostatic discharge.
2. Description of the Related Art
The aggressive scaling of Complementary Metal Oxide Semiconductor (CMOS)-based technologies is resulting in transistors with smaller channel lengths, thinner gate oxides and shallower and more abrupt junctions. As a result, the semiconductor devices have smaller junction and gate oxide breakdown voltages and smaller parasitic bipolar trigger voltages. This results in increased susceptibility of the internal circuits which are comprised of these transistors to an electrostatic discharge (ESD) event.
ESD events may be generally characterized as Human Body Model (HBM), Charged Device Model (CDM) or Machine Model (MM) events. The direct transfer of electrostatic charge from a human body to a semiconductor device is often referred to as a human body model event (HBM). A discharge can also occur from a charged conductive object, such as a metallic tool or fixture; this mode is referred to as the Machine Model. Another cause of ESD is the transfer of charge from a device, for example, if a device itself becomes charged and discharges via any pin that is grounded. This is often referred to as a charged device model event (CDM). CDM pulses have a rise time in the order of hundred's of picoseconds and a pulse width of ins.
ESD protection devices such as Silicon Controlled Rectifiers (SCR's) are connected to Input/Output (I/O) pads to safely discharge the ESD current to ground without damaging internal circuits. The following are exemplary prior art references that have proposed the use of silicon controlled rectifiers (SCRs) as a means of protecting CMOS devices for ESD: U.S. Patent Application Publication No. U.S. 2002/0079538 (incorporated herein by reference); U.S. Patent Application Publication No. U.S. 2002/0163009, Ker et al, Nov. 7, 2002 (incorporated herein by reference); U.S. Patent Application Publication No. U.S. 2003/0075726, Ker et al., Apr. 24, 2003(incorporated herein by reference); and, U.S. Patent Application Publication No. U.S. 2004/0207020, Chen et al, Oct. 21, 2004 (incorporated herein by reference). While SCR's are effective ESD protection devices for HBM and MM ESD discharges, they suffer from a large turn-on time compared to grounded-gate NMOSFETs or diodes making them not so effective for very fast ESD pulses such as CDM discharges. Due to the significant advantages of SCR devices, it is essential to improve the turn-on times of the SCR to enable effective ESD protection under all types of ESD discharges. The present invention addresses the issue of faster turn-on times for silicon controlled rectifiers.